The present invention claims priorities from Japanese Patent Applications No. 9-306014 filed Sep. 7, 1997, No. 10-161142 filed Jun. 17, 1998 and No. 10-169068 filed Jun. 17, 1998, which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a high frequency filter formed in a high frequency integrated circuit or a micro wave integrated circuit (MIC) formed by mounting or forming a plurality of circuit elements on a substrate. The present invention can be utilized in a communication equipment, a radar equipment, a measurement device, etc. Although the present invention was developed for the purpose of utilization in a high frequency circuit having an operating frequency exceeding 10 GHz, the operating frequency is not limited thereto and the present filter can be used in a circuit having an operating frequency lower than 10 GHz.
2. Description of Related Art
A micro wave integrated circuit (MIC) having a dielectric resonator mounted on a dielectric substrate of such as ceramics has been known. For example, Japanese Patent Application Laid-open No. Hei 6-334413 discloses a construction of such micro wave integrated circuit in which an insulating film is formed on a surface of a semiconductor substrate, a conductor line is formed on the insulating film and a dielectric resonator is mounted in the vicinity of the conductor line by as adhesive.
Japanese Patent Application Laid-open No. Sho 61-288486 discloses a technique by which a hole formed in an alumina substrate before baking is filled with ferrite and the ferrite is fixed to the hole of the substrate by burning. This technique is useful in mounting the ferrite on the ceramic substrate. Although the present invention belongs to this technique, this prior art technique is merely to mount the ferrite unit on the alumina substrate and include neither a technical thought that a plurality of mutually related circuit parts on an alumina substrate to unify them as an integrated circuit nor a technical thought of mounting dielectric parts on the same substrate. Further, the prior art technique does not include a technical thought of use of hard ferrite as the ferrite and magnetization of the hard ferrite correspondingly to an electrical characteristics of the circuit.
On the other hand, a high frequency band exceeding 10 GHz have been used in various devices recently. Particularly, a technique for mass-producing inexpensive high frequency filters which have uniform characteristics and can be stably utilized in a 60 GHz band or a 70 GHz band assigned to a radar device for automobile or a distance measuring device has been required. Further, with an improvement of the ferrite technology, a low loss hard ferrite whose anisotropic magnetic field can be regulated and which has high coercive force has become usable.
Such technique for mounting a dielectric resonator in the vicinity of the conductor line is basically superior technique. However, since such technique utilizes the adhesive, the number of manufacturing steps for producing such high frequency filters is increased and the characteristics thereof tend to vary, causing the prior art technique to be not always suitable for mass-production. Further, in a case where a filter circuit and an antenna circuit for use in such frequency band as high as several tens GHz are designed, circuit parts to be mounted on a circuit substrate, for example, a dielectric part in the form of a circular rod, may have a diameter in the order of 2 to 3 mm and magnetic parts may become smaller than the dielectric part. In order to correctly adhere parts having such size to the substrate, a machining preciseness of the parts as high as several tens microns is required. In order to achieve such high machining preciseness, a high precision machine tool is necessary, so that a considerable investment is required in mass-producing such circuits.
It has been found experimentally that the technique for mounting ferrite on an alumina substrate before baking thereof is very effective for ferrite parts having diameter of 2 mm. However, it is impossible to design a desired filter circuit or an antenna circuit operable in the frequency band of several tens GHz with using such ferrite part unit as disclosed in Japanese Patent Application Laid-open No. Sho 61-288486. If a plurality of such parts are arranged on a single substrate and connected each other to form a circuit, a large number of working steps are required and the loss due to the connections and the reliability of connecting points, etc., become problems. Therefore, it is necessary to provide an integrated circuit by integrally incorporating micro strip lines using conductors having similar size to that of the ferrite unit and dielectric parts on one and same substrate.
When a high frequency filter to be used in the frequency band of several tens GHz by using a micro wave integrated circuit, the size of the substrate becomes very small. In an example, the size is 8 mm.times.3 mm.times.0.3 mm. In such small size circuit, the electric characteristics thereof is substantially influenced by even minute size error of the part and it is indispensable to regulate the frequency characteristics thereof after manufacture thereof. In the prior art technique, such regulation depends largely upon experience and is not suitable to manufacture uniform products. Further, the reproducibility thereof is not enough. In addition, when the regulation is performed by mechanically cutting the magnetic part, the regulation can be made only unidirectional. Since, when the part is over cut, it is impossible to recover the over-cut portion, the yield becomes low.